This is called in the schools proportio habitudinis
(St Thomas's phrase here), sed non existentiae. I
have written elsewhere: "There is an analog
between the paper plan of the building and the
building as it exists. . . . It is obvious that plan
and building do not receive the same name in the
same sense: yet there is some connexion and relation
between the two, a relation of the less to the
incomparably greater which it somehow exhibits and
represents" (Oxford and Cambridge Conferences, second
series, pp. 132, 133). Another, and possibly a more
apt illustration, might be supplied by modern 'graphs,'
I mean one of those 'curves of temperature,' or the
like, which correspond to, but do not (except
a very indirect or highly generic fashion) resemble
the facts which they truly represent.